I haven't seen a chart and I doubt one exists due to the fact that the systems change every day, many different versions exist etc etc. You then have to have someone that knows each system intimately so that the information is factual.
Then you have other factors that are not necessarily technical factors e.g.
Number of developers working on product
Company's commitment to product
Company's commitment to regular releases
Company's commitment to release dates to new features
Company's commitment to Community and Open Source as a whole.
Community involvement in the product
Documentation, Blogs, FAQ's, recent documentation etc
Some of the items above also then contribute to the personal bias towards a product.
Some of it is reading a large amount of posts from each of the forums to gain an understanding of why people go for one product or another.
I have personally tried all the main ones out there. I have been in the VoIP/Comms space for 5-6 years, so I have seen what has gone on and it makes it easy to make a decision.
I select the Elastix product as they are taking a professional approach to the system as a whole (training, resellers, business features in the product, a good number of internal developers, a good interest in the community, marketing of the name, marketing of the product). Furthermore, the feature set now and what is coming in 2.0 is way past what the others have done or are proposing.